1. Field of the Invention
The present invention relates to document feeders and document processing. More specifically, the present invention relates to systems and methods for detecting a document trailing edge during the feeding and transporting of documents.
2. Background Art
A typical system for feeding and transporting documents includes a feeder and a separator in the document-feeding portion of the system, and a series of roller pairs or belts in the document-transporting portion of the system. In the feeding portion of the system, the feeder acts with the separator to feed documents singly, in order, from a stack. In the transporting portion of the system, the roller pairs and/or belts convey the documents, one at a time, past other processing devices such as readers, printers, and sorters that perform operations on the documents. The feeder is typically a feed wheel, but may take other forms. The separator may be a wheel, but also may take other forms such as a belt. Further, the components in the transporting portion of the system may take a variety of forms. The systems also include a component in the document-feeding portion of the system that nudges documents into the nip between the feeder and the separator. A suitable nudger may be a nudger wheel, but may take other forms. An existing document feeder is shown in U.S. Pat. No. 6,199,854. That patent describes a document feeder with a variable speed separator.
In existing systems for feeding and transporting documents, operations that depend on the position of the document are generally performed in the transport stage, or transporting portion of the system. For example, U.S. Pat. No. 5,848,784 describes a document separation apparatus. That patent describes the downstream acceleration/deceleration of documents with pinch rollers to adjust document spacing. U.S. Pat. Nos. 5,419,546, 5,437,375; 5,439,506; 5,509,648; 5,671,919; and 5,908,191 describe examples of other document operations.
Those skilled in the art will understand the importance of detecting the leading and trailing edges of documents, and the gaps between them, as they pass through the feeding system and the transport system beyond. Many operations to be performed upon the documents (e.g., printing, reading, imaging and so forth) are required to be performed at specific locations along the length of the document, and so it is very important for the system to be able to detect when the leading or trailing edge of a document appears at a specific point. From this data, the system can extrapolate its necessary understanding of where the document is, how fast it is traveling, and when and where specific operations should be performed upon it.
Similarly, it is just as important for the system to understand the lengths and locations of gaps between documents as it is for it to understand the lengths and locations of the documents themselves. It will be understood that document-processing systems seek to produce the highest possible throughput rates, and therefore, workers seek to minimize gaps between successive documents. At a given transport speed, a gap is a unit of time in the operation of the system which is not occupied by a document, and is therefore lost to productive processing. At the same time, systems often require a discrete and controlled time interval between documents, e.g., to transmit data gathered from the previous document, or to reset mechanisms after processing the previous document, and the optimum gap is usually dependent upon the length of the previous document. The longer the previous document (generally speaking) the longer the gap required after it before the system can be ready to commence processing the next document.
Operators therefore always seek to reduce the gaps between documents to the smallest possible consistent with all system functions, and for system functions, gaps are most usually dealt with as time measurements rather than measurements of physical distance. In order to best measure and manage both document lengths and the gaps between them for the optimum throughput, workers will understand that it is advantageous to be able to detect both leading and trailing edges of documents as early in their processing as possible, and preferably, during the feeding process, before any other processes are to be performed upon them.
Ideally, such a system would measure the position of the edges of the document in the feed hopper even before it is fed. However, documents can vary widely in overall length. For example, if one considers a high-speed document processing machine such as the commercially available Unisys NDP2000 (Unisys Corporation, Unisys Way, Blue Bell, Pa., 19424) the specified range of document length is 4.25″–9.25″, or a range greater than 100% between the shortest and longest. In order to meet this need, a detector capable of detection over a wide possible range of document trailing edge positions would be required.
In order to perform operations on documents that depend on document position, leading and/or trailing document edges are detected depending on the operation to be performed. A known device for detecting document edges is the photo edge detector. U.S. Pat. No. 5,848,784 describes the use of an edge detector. The edge detector is suitable for some applications, but may be sensitive to, for example, printing on the documents and/or document thickness and/or opacity.
Those skilled in the art will understand that photo-edge detectors can be and are used to detect both leading and trailing edges of documents, but they can only function upon individual documents, e.g., when traveling singly in a document track. Since they rely for their function upon the interruption of a beam of light, they are unsuitable for use in a feed hopper that contains many documents. Such sensors have been used to detect leading edges of documents as they leave the stack in the feed hopper, but cannot be used to detect trailing edges until the trailing edge has entirely separated from the stack of documents behind it in the feed hopper.
For the foregoing reasons, there is a need for an improved system and method for feeding and transporting documents that detect a document trailing edge at a known location while it is still within the feed hopper.